This invention relates to an apparatus and method for monitoring the stitching quality of sewing machines and, in particular, to detecting skipped and malformed stitching for Class 400 chainstitch sewing machines.
With the clothing industry becoming increasingly automated, there is a need for systems that monitor and regulate the functions and output of high speed sewing equipment. Certain of these systems are utilized to monitor the stitching of sewing machines to detect malformed or skipped stitching in apparel manufactured by Class 400 chainstitch sewing machines. The Class 400 chainstitch is employed in a wide range of areas within the apparel industry because it provides a fast, economical, resilient, and strong stitch chain. The Class 400 stitch tends to be very elastic and is well suited for seaming operations, for example, inseaming pants and closing synthetic bags, on wovens and knits of many types and weights of materials. However, in Class 400 chainstitch, malformed or skipped stitching tend to weaken the entire stitch chain and, as a result when included in the final product, the defective product may prematurely fail, for example by unraveling.
The 400 Class "multi-thread chainstitch" is formed by a sewing machine passing one or more needle thread loops through the material. Those needle thread loops are interlooped on the underside with a looper thread supported on a looper As an exemplary Class 400 chainstitch, stitch type 401 is formed with two threads, the needle thread and the looper thread. An angularly reciprocal looper, located underneath the material, engages the needle loop projected by an axially reciprocal needle underneath the material. The looper retains the needle loop when the needle is retracted and, in addition, draws the looper thread from the previous stitch through the needle loop. The needle then penetrates the material again between the looper thread and the previous needle loop. As a result, when the looper retracts, the needle thread, which comprised the needle loop, tightens and thus completes a stitch. A more detailed description of the chainstitch type 401 is provided in Union "Special Stitch Formation Type 401" brochure, published by Union Special Huntley, Illinois (1979).
In the general use of chainstitch type 401 sewing machines, improper stitches maY from time to time be introduced in a workpiece. Generally, improper stitches may have the form of malformed stitches or skipped stitches. Collectively, these malformed and skipped stitches are referred to as "improper stitches" hereinbelow. There are many causes of improper stitches. Malformed stitches can develop from improper synchronization between the active elements within the sewing machine and the needle and looper thread loops. In particular, the malformed stitches are formed when the needle thread loop around the blade of the looper is improperly positioned and as a result the needle on its downward travel can enter this loop, forming a "101-type" stitch.
In general, skipped stitches also result from improper synchronization of the needle thread loop and the looper thread loop and may also occur from deflection of the needle. There are primarily two types of skipped stitches: the "needle loop" skip and "triangle" skip. The needle loop skip develops when the looper fails to enter the needle loop and as a result the upward motion of the needle, the needle thread controls, and the feed motion pull the loop to the top of the fabric The triangle skip is formed not by the looper failing to enter the needle loop, but when the needle fails to enter the looper loop. Consequently, since the needle loop was picked up by the looper, the needle thread remains in the material or is loose on the top side of the fabric.
In the prior art, skipped stitch detection systems are based upon monitoring the tension of the looper thread and/or the needle thread. As detailed in Rockerath et al., U.S. Pat. No. 4,102,283, the loss of thread tension generally said to correspond to a skipped stitch and this reduction in normal thread tension triggers a sensing device. The sensitivity of these systems ranges from complete loss of thread tension, for example, due to the thread breaking, to sensing a momentary reduction in the normal thread tension.
A primary shortcoming of the prior art is the unreliability of these systems at high sewing speeds, for example, greater than 9,000 stitches per minute. These systems fail to detect a momentary reduction of thread tension when the sewing machine is operating at high sewing speeds. The reduction in tension for a malformed or skipped stitch at high sewing speeds tends to be less and in a range that the prior art fails to detect As a result, these systems tend to be less reliable and thus fail to perform these functions with great accuracy.
There exists a need for better methods and systems that are reliable at high sewing speeds. To accommodate the advances in the clothing automation, particularly the increase in sewing speeds, a simple, reliable system for monitoring the malformed and skipped stitches would satisfy a substantial need in the art.